xlsx export: remove redundant code

[matthewhegarty]

This PR removes what I believe is a redundant check from XLSX export dset_sheet(). It provides a minor simplification of the code.

If we check the source for openpyxl.cell.cell.Cell, the cell value setter calls _bind_value(). This method can raise a ValueError but not a TypeError. Therefore I have removed the checks for TypeError, and I assume that if this does occur somehow, then we would want this raised and not caught because this might be masking an error the user might be interested in.

I also removed catching TypeError when str(col) is called for the same reason.

I have added tests to increase the coverage.

[codecov[bot]]

Codecov Report

Merging #541 (61ae056) into master (bff435d) will increase coverage by 0.11%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #541      +/-   ##
==========================================
+ Coverage   91.28%   91.39%   +0.11%     
==========================================
  Files          28       28              
  Lines        2719     2720       +1     
==========================================
+ Hits         2482     2486       +4     
+ Misses        237      234       -3     

Impacted Files	Coverage Δ
src/tablib/formats/_xlsx.py	100.00% <100.00%> (+2.97%)	:arrow_up:
tests/test_tablib.py	98.73% <100.00%> (+<0.01%)	:arrow_up:

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